1. Field of the Disclosure
The invention generally relates to electrical switches and more specifically to double pole-double throw proximity switches for use in nuclear environments.
2. Related Technology
Nuclear reactors require robust control systems to ensure that the nuclear reaction can be shut down in any emergency. In extreme emergencies, such as containment vessel breaches, nuclear reactors can experience a loss of cooling fluid, which could potentially lead to a runaway nuclear reaction (i.e., nuclear meltdown). In such situations, the control systems of the nuclear reactor must be capable of shutting down the nuclear reactor regardless of damage or other non-normal operating conditions. To ensure that the control systems are capable of shutting down the nuclear reaction in any situation, control system components are subject to rigorous testing conditions.
One component of control systems is a proximity or limit switch. The proximity or limit switch may use magnetic attraction to complete various electronic circuits based on the proximity of a target. For example, during a loss of coolant, a coolant level sensor (such as a float sensor) may gradually approach the proximity switch. When the sensor reaches the sensor's maximum detection range, the sensor may complete various electronic circuits that indicate loss of coolant and/or that begin a shutdown sequence for the nuclear reactor. These proximity switches must be capable of detecting the target during extreme conditions. As a result, many regulatory agencies require proximity switches used in nuclear operations to pass rigorous tests. One of these tests is a seismic test where the proximity switch is subject to violent accelerations of up to 10 g to simulate conditions in an earthquake. The proximity switch must survive the seismic testing with no contact discontinuity.
One type of switch that has been shown to pass the rigorous tests required for nuclear operation is a high amp rated mechanical switch, one of which is manufactured by NAMCO®. However, these high amp rated mechanical switches suffer from various drawbacks, such as low performance at low current (due to resistance problems), requiring a lever arm that connects to the target, complicated internal moving parts (e.g., springs, cams, and the like), multiple points of potential contamination ingression, supplementary repair and regular maintenance of the internal moving components, and a short service life, generally less than about 9 years.